1. Field of the Invention
The present invention relates to a CMOS image sensor and fabricating method thereof. The present invention is suitable for a wide scope of applications, and is particularly suitable for enhancing a light condensing effect by providing an inner microlens on a semiconductor substrate.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts an optical image to an electric signal. In a CCD (charge coupled device) image sensor, a plurality of MOS (metal-oxide-metal) capacitors are arranged close to one another to transfer and store electric charge carriers. In a CMOS (complementary MOS) image sensor, a plurality of MOS transistors corresponding to a number of pixels and are fabricated by CMOS technology. CMOS technology uses a control circuit and a signal processing circuit as peripheral circuits, and a switching system of detecting outputs step by step using the MOS transistors.
The CMOS image sensor includes signal processing chips including photodiodes. CMOS image sensors are advantageous in their degree of integration since an amplifier, an analog/digital (A/D) converter, an inner voltage generator, a timing generator, a digital logic, etc. can be integrated on each of the chips. The CMOS image sensor is also advantageous in power and cost reductions. Moreover, mass production of the CMOS image sensor is enabled through an etching process of a silicon wafer that is cheaper than a fabricating process of a CCD. The CCD is fabricated through a special process. Thus, the image sensor has expanded into application fields such as digital cameras, smart phones, PDAs (personal digital assistants), notebook computers, security cameras, barcode detectors, and toys.
As the size of CMOS image sensors reduces because of high degrees of integration, the pixel size is decreased. Thus, a fill factor, which indicates a rate of a pixel area within a chip, generally becomes only approximately 30-40%. Thus, photosensitivity cannot be maximized. A method of increasing the fill factor has been proposed to raise the photosensitivity, however, a logic circuit part for signal processing still puts limitations on the corresponding area. Hence, a microlens (ML), which includes a photoresist layer to enable maximum light absorption with a photodiode, condenses light by diverting a path of the light incident on an area except the photodiode. The microlens is mainly used to maximize the intensity of the incident light.
A method of fabricating a CMOS image sensor with a microlens according to a related art is explained with reference to FIG. 1 as follows.
Referring to FIG. 1, a plurality of epitaxial layers (not shown) may be formed on a semiconductor substrate 10. A plurality of photodiodes 18 are formed on the epitaxial layer. In particular, a first epitaxial layer (not shown) may be grown on the semiconductor substrate 10, a red photodiode (not shown) may be formed on the first epitaxial layer, a second epitaxial layer (not shown) may be grown on the first epitaxial layer including the red photodiode, and a green photodiode (not shown) may be then formed on the second epitaxial layer.
A third epitaxial layer (not shown) may be grown on the second epitaxial layer including the green photodiode, a blue photodiode (not shown) may be formed on the third epitaxial layer, and a trench for field isolation is formed on the third epitaxial layer. An STI (shallow trench isolation) layer 11 is then formed by filling the trench with an insulating material.
An insulating interlayer 13 is formed on the third epitaxial layer, a first metal layer (not shown) may be formed on the insulating interlayer 13, and a metal line 14 is then formed by patterning the first metal layer. The process of forming the insulating interlayer 13 and the metal line 14 is repeated several times to stack the interlayer 13 and the metal line 14.
Then, a color filter layer 15 is formed on the insulating interlayer 14. A device protecting insulating layer 16 is formed on the color filter layer 15 to protect a device from moisture or physical shock. A microlens 17 is then formed on the device protecting insulating layer 16.
However, when fabricating the CMOS image sensor according to the related art, the microlens 17 formed on a top layer is dependent on a focal length between the semiconductor substrate 10 and the top layer. Hence, a light refracted by a lateral side of the microlens 17 having a long focal length does not enter the photodiode. Instead, the light enters a neighboring pixel that results in optical crosstalk between neighboring pixels.